Aitkin County was established in 1857 as Aiken County and had a spelling change of its name to the current spelling in 1872. Aitkin County was named after William Alexander Aitken, a fur trader for the American Fur Company, under John Jacob Astor. Formed from Ramsey and Pine counties, Aiken County originally consisted of the 17 townships closest to Mille Lacs Lake. It acquired out-lands of Ramsey, Itasca and Pine Counties to its north and east. It was organized in 1871, taking up lands from Cass and Itasca Counties and losing a point located in the southwestern corner to Crow Wing County to form its current boundaries.

In recent years, average temperatures in the county seat of Aitkin have ranged from a low of 0 °F (−18 °C) in January to a high of 80 °F (27 °C) in July, although a record low of −47 °F (−44 °C) was recorded in January 1972 and a record high of was recorded in August 1976. Although these records are the official records, temperatures above 100 °F (38 °C) has been detected throughout Aitkin County and surrounding areas. Average monthly precipitation ranged from 0.79 inches (20 mm) in February to 4.46 inches (113 mm) in June.[6]

There were 6,644 households out of which 22.60% had children under the age of 18 living with them, 57.50% were married couples living together, 6.30% had a female householder with no husband present, and 32.90% were non-families. 28.70% of all households were made up of individuals and 14.00% had someone living alone who was 65 years of age or older. The average household size was 2.28 and the average family size was 2.76.

In the county the population was spread out with 20.90% under the age of 18, 5.50% from 18 to 24, 21.60% from 25 to 44, 29.10% from 45 to 64, and 23.00% who were 65 years of age or older. The median age was 46 years. For every 100 females there were 101.60 males. For every 100 females age 18 and over, there were 99.90 males.

The median income for a household in the county was $44,139, and the median income for a family was $58,290. Males had a median income of $51,604 versus $30,633 for females. The per capita income for the county was $21,848. About 5.20% of families and 7.60% of the population were below the poverty line, including 15.50% of those under age 18 and 11.00% of those age 65 or over.